It’s a difficult and potentially volatile question because it begs much bigger questions: What is a good education? And what should children get out of school? In today’s data-driven culture, policy debates about education are narrowly focused on what can be measured: namely, how students do on standardized tests. This is why there is so much focus on using tests to evaluate teachers. Add to this that the most provocative and influential research about teacher quality is being done by economists, whose currency is stuff that can be measured and counted.

Teaching is too complex to be fully captured by a test score. Even Eric Hanushek, the economist who pioneered the idea of using tests to compare teachers, says no single measure can do teaching justice. “I think a good teacher has a multitude of skills and has figured out how to balance all kinds of things,” he says. “[Teaching] is much more complicated than the researchers and the educational institutions have been able to figure out.”

That’s the problem with the debates about teachers now. Economists have come up with all kinds of compelling evidence showing how crucial teachers are, and how much teaching quality varies from classroom to classroom. But researchers still can’t explain exactly what it is that effective teachers do to raise student achievement. And, in the rush to start judging teachers by how much their students’ scores go up, the conversation about what good teaching is, and how teachers learn to be good, is getting lost.

About a year ago, I began work on a radio documentary called Testing Teachers. One of my goals was to better understand what good teaching looks like. I observed teachers, asked them about their methods, and interviewed a lot of experts. I can’t say I figured out exactly what good teaching is, but my experience and observations changed the way I think about the question.

First, using test scores to judge teachers focuses attention on what individual teachers do to improve student learning. In the best schools I visited and heard about, students learn from many different teachers over the course of a school year. Whether through team- teaching, specialists, or remediation classes, students are exposed to lots of adults each day, and their learning is the cumulative effect of all those people’s efforts. A good education is a collaboration among teachers in a school. Focusing narrowly on individual teachers obscures this critical element of effective schooling.

Second, there is too much attention paid to the idea that good teachers possess innate skill or talent, and not enough attention on the idea that good teaching is something that can be taught. The question of whether teachers are born or made is a core tension in this debate. If researchers understood more about good teaching, there might be less focus on what kinds of people will become good teachers and more effort put into how to teach the skills of effective teaching to more people.

Third, as school systems nationwide begin to develop new ways to evaluate teachers, there’s a danger that the complex craft of teaching will be reduced to a checklist. There are many ways to teach effectively. Good teaching is not something that can be standardized. Teachers should be judged by what their students learn. Tim Daly, president of The New Teacher Project, an organization that recruits and trains teachers, said something that stuck with me: One way to think about good teaching is to think about what makes a good comedian.

“Is it a person that tells jokes in a certain way?” he asks. “Is it somebody that is very racy and uses a lot of profanity? Is it somebody that talks about everyday things or tells long stories?” No, Daly says. “We can think of people that are considered very good comedians who do it every different manner of way. The universal is that people laugh.”

He says the same holds for good teaching—the universal is that students should learn. That’s why Daly supports the idea of using test scores to evaluate teachers; tests are evidence of student learning. However, he, and every other expert I spoke with, says tests are a less-than-ideal way to measure teachers. But, tests are still one of the few measures schools have.

Fourth, teaching has always been complex, but it’s arguably more complicated and more difficult now. Scientists and education researchers have learned a huge amount in recent years about how people learn. That knowledge puts new demands on teachers. It’s no longer enough to stand and lecture at the chalkboard, as studies show a lot of students don’t learn well this way. Good teachers design lessons that tap into the many ways people learn. They invent projects and group activities to get students thinking and talking.

“What kids say to teachers, that’s kind of like the raw material of teaching [now],” says Heather Hill, an education researcher at Harvard University. “Kids come up with funky ways of doing things. They are wildly inventive. And teachers have to deal with that.”

This leads to my final point: Complex work demands complex evaluation. Using test scores is, at best, a simple tool. The nation needs more ways to gauge what students learn in school, and this requires a deeper conversation about the purpose of education.

As part of the documentary Testing Teachers, my colleagues and I are asking listeners and web users to tell us about the greatest teachers they’ve had, and what made those people stand out. No one writes about how a teacher raised test scores. They do, however, have fascinating things to say about good teaching. Perhaps more importantly, they have a lot to say about what good learning is.

When schools evaluate teachers they should find out what students say about teachers, and what they think would make their teachers better.

Illustration by Will Etling

Emily Hanford is a reporter for American RadioWorks, the national documentary unit of American Public Media, and the creator of the radio programTesting Teachers.